It is known to provide a material handling system that includes vacuum cups or the like that are adapted to be moved into engagement with an object, such as a substantially flat object or panel or the like, and to lift and move the object to a desired location. Such vacuum cups or suction cups may be moved into engagement with the object, and a vacuum source may be actuated to create a vacuum between the object and the cup such that the object is retained to the cup as it is transported to the targeted area. An example of such a vacuum cup is disclosed in U.S. Pat. No. 4,662,668, which is hereby incorporated herein by reference. The vacuum cup may include a proximity sensor that determines when the cup has engaged an object and that the object is positioned adjacent to the end of the proximity sensor.
Often, such material handling systems are used to move objects that may have different thicknesses from one end of the object to the next, such that as multiple objects are stacked on top of one another, the angle between the thinner sides and the thicker sides may increase as the stack height of the objects increases. A particular example of such an application is for automotive doors, where one end of the door may be a substantially flat piece of sheet metal, but the other end may have reinforcement metal or other structures, such as at the hinge area of the door. When a door is laid down in the assembly plant, the thicker hinged end of the door may be higher than the thinner opposite end of the door, and that difference in height (and thus of the surface angle of the door) may be increased as multiple doors are stacked upon one another in the same orientation. Accordingly, as each door is removed from the stack, the next lower door that is to be removed may be at a different angle than the one that was previously removed.
It has been proposed to accommodate such variations in the objects by providing a linear actuator or cylinder at each vacuum cup, such as disclosed in U.S. Pat. No. 4,129,328 issued to Littell. The cylinder may extend and retract to individually move the respective vacuum cup into engagement with the surface of the object. Although such a system may accommodate variations in the surface of the object, the system may include control circuitry and multiple actuators, which may add significantly to the cost and complexity of the system.
Therefore, there is a need in the art for a material handling system that engages and picks up and moves objects that have curved or angled or uneven surfaces and that overcomes the shortcomings of the prior art.